7.2.84
अष्टन आ विभक्तौ
Operational rule vidhi
The word अष्टन् gets an optional आकारादेश when followed by a हलादि विभक्तिप्रत्यय.
Word by word (padaccheda)
Target अष्टनः ṣaṣṭhī · ekavacanaSubstitute / term आः prathamā · ekavacanaRight context विभक्तौ saptamī · ekavacana
- Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
- Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
- Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
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Full reading
अष्टनः आः विभक्तौ
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
अष्टनः → आः / विभक्तौ
Replace aṣṭanaḥ with āḥ when vibhaktau follows. Inherited words (anuvṛtti) may add further conditions.
Commentary
आ is substituted for the final of अष्टन् before a case-ending.
Thus अष्टाभिः, अष्टाभ्यः, अष्टानाम्, अष्टासु ॥ Why 'before a case-ending'? Observe अष्टत्वम्, अष्टता ॥ The rule of this sutra is an optional one, and we have in the alternative अष्टभिः, अष्टभ्यः ॥ The आ in the sutra indicates the individual letter आ, and not आ belonging to the general class आ ॥ For the generic आ would include the nasalised आं also, and as the letter replaced (अन्) is a nasal, the substitute would have been also nasal आँ, but it is not so. See 6.1.172 and 7.1.22. This rule applies also when the word stands at the end of a compound, as प्रियाष्टानः or प्रियाष्टौः 7.1.22. The word विभक्ति governs the subsequent sutras upto 7.2.114.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.